Soy Isoflavones Aid Prostate Cancer Fight

ST. PAUL, Minn.—Greater intake of soy protein isolate with isoflavones may increase excretion of certain estrogen metabolites linked to promotion of hormone-related cancers, according to a study coordinated out of the University of Minnesota (J Nutr. 2007;137:2258-63). Researchers sought to determine the effects of soy protein isolate consumption on estrogen metabolism in men at high risk for developing advanced prostate cancer. In the randomized, controlled trial, 58 men supplemented their regular diets with one of three protein isolates: isoflavone-rich soy protein isolate (SPI+) with 107 mg isoflavones/d; alcohol-washed soy protein isolate (SPI-) with less than 6 mg isoflavones/d; or milk protein isolate (MPI). Each intervention provided 40 g/d of protein. The urinary estrogen metabolite profile was measured at baseline, and three and six months of intervention.

Both soy groups had higher excretion of urinary estradiol (E2) than the MPI group during supplementation. After six months, the SPI+ group had a significantly higher ratio of urinary 2-hydroxy estrogens to 16-hydroxyestrone (2:16 OH-E1) than the MPI group. These findings were consistent with studies in postmenopausal women showing greater urinary excretion of the estrogen metabolites—thus, a decreased risk of hormone-related cancers—as well as epidemiological studies that found significantly lower excretion of E2 and a lower 2:16 OH-E1 ratio were reported in prostate cancer cases compared to controls.

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